Composite Output And Subcarrier Input - Orban Optimod-FM 8300 Operating Manual

2-8
INSTALLATION

Composite Output and Subcarrier Input

The digital input clip level is fixed at 0 dB relative to the maximum digital
word. The maximum digital input will make the 8300 input meters dis-
play 0 dB. The reference level is adjustable using the
The 8300 is a "multirate" system and its internal sample rate is 32 kHz
and multiples thereof (up to 512 kHz). The output is strictly band-limited
to 16 kHz. Therefore, the output can pass through a 32 kHz uncom-
pressed link with bit-for-bit transparency. Because sample rate conversion
is a phase-linear process that does not add bandwidth, the 8300's output
signal will continue to be compatible with 32 kHz links even if it under-
goes intermediate sample rate conversions (for example, 32 kHz to 48
kHz to 32 kHz).
There are two composite outputs. These carry the encoded stereo signal, the ste-
reo pilot tone, and any subcarriers that may have been applied to the 8300's subcar-
rier inputs.
Each output's level is independently adjustable from –12.3 dBu to +12.0 dBu.
The output impedance of composite output #1 and composite output #2 can be set
to 0 or 75 via jumpers J2 and J3 respectively (located on the I/O Board). As
shipped, the link is on pins 3 and 4, yielding 0 impedance. To reset a given output
to 75, place the link on pins 1 and 2 of its associated jumper. (See the schematic on
page 6-49 and the parts locator diagram on page 6-46.)
Each output can drive up to 75 in parallel with 0.047F before perform-
ance deteriorates significantly (see Figure 2-3 on page 2-9). A G
switch is available on the rear panel. This is useful to prevent ground
loops between the 8300 and the transmitter.
Connect the 8300's composite output to the exciter input with up to 100 feet
(30.5m) of RG-58 / U or RG-59 / U coaxial cable terminated in BNC connectors.
Longer runs of coax may increase problems with noise, hum, and RF
pickup at the exciter. In general, the least troublesome installations place
the 8300 close to the exciter and limit the length of the composite cable
to less than 6 feet (1.8m).
We do not recommend terminating the exciter input by 50 or 75
unless this is unavoidable. The frequencies in the stereo baseband are
low by comparison to RF and video, and the characteristic impedance of
coaxial cable is not constant at very low frequencies. Therefore, the
transmission system will usually have more accurate amplitude and phase
response (and thus, better stereo separation) if the coax is driven by a
very low impedance source and is terminated by greater than 1k at the
exciter end. This also eases thermal stresses on the output amplifier in the
stereo encoder, and can thus extend equipment life.
If the Orban CIT25 Composite Isolation Transformer is used, the exciter
must present a 1k or greater load to the transformer for proper trans-
former operation.
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